1. Field of the Invention
The present invention relates to a touch module, especially to a touch module having a dynamic capacitance matching mechanism.
2. Description of the Related Art
Please refer to FIG. 1, which illustrates a touch detection circuit of a prior art capacitive touch module. As illustrated in FIG. 1, the touch detection circuit includes a comparator 100, a touch sensing electrode 110, a transferred-charge-storing capacitor 120, and a reset switch 130.
The comparator 100 has a first input end, a second input end, and an output end. The first input end is used to couple with the touch sensing electrode 110, the transferred-charge-storing capacitor 120, and the reset switch 130; the second input end is coupled with a reference voltage Vth; and the output end is used to provide an output signal Vout.
The touch sensing electrode 110 has a first capacitance. When a finger touches the touch sensing electrode 110, the touch sensing electrode 110 will exhibit a second capacitance, wherein the second capacitance is larger than the first capacitance. During a period when the touch sensing electrode 110 is connected with a DC voltage VDD, the quantity of charge accumulated on the touch sensing electrode 110 will be larger with the touch sensing electrode 110 exhibiting the second capacitance than with the touch sensing electrode 110 having the first capacitance.
The transferred-charge-storing capacitor 120, having a capacitance much larger than the first capacitance or the second capacitance, is used to receive charge transferred from the touch sensing electrode 110.
The reset switch 130 is used to clear the charge on the transferred-charge-storing capacitor 120 after the voltage on the transferred-charge-storing capacitor 120 reaches the reference voltage Vth.
Please refer to FIG. 2(a)-2(c), which illustrate an operation procedure of the circuit in FIG. 1. FIG. 2(a) illustrates a charging stage, where a DC voltage VDD is used to charge an equivalent capacitor of the touch sensing electrode 110. FIG. 2(b) illustrates a charge transfer stage, where the touch sensing electrode 110 is connected with the transferred-charge-storing capacitor 120 to form a loop to transfer charge from the touch sensing electrode 110 to the transferred-charge-storing capacitor 120.
FIG. 3 illustrates a waveform diagram of a voltage VA on the transferred-charge-storing capacitor 120, and the output signal VOUT of the comparator 100. As illustrated in FIG. 3, the time for VA to reach Vth is longer when no touch event occurs than when a touch event takes place.
However, when the area of the touch sensing electrode 110 needs to change dynamically to increase sensing efficiency, the structure of FIG. 1 can cause touch detection errors. For example, when the area of the touch sensing electrode 110 is corresponding to a touch plane to serve as a switch, the structure of FIG. 1 can falsely determine a touch event is happening even though no touch event is actually taking place—the reason is that the time for VA to reach Vth will be greatly reduced in this scenario to cause a false touch detection.
To solve the foregoing problems, a touch module having a dynamic capacitance matching mechanism is needed.